EP0554388A1 - Resonateur de volume rf pour resonance magnetique nucleaire. - Google Patents

Resonateur de volume rf pour resonance magnetique nucleaire.

Info

Publication number
EP0554388A1
EP0554388A1 EP91920669A EP91920669A EP0554388A1 EP 0554388 A1 EP0554388 A1 EP 0554388A1 EP 91920669 A EP91920669 A EP 91920669A EP 91920669 A EP91920669 A EP 91920669A EP 0554388 A1 EP0554388 A1 EP 0554388A1
Authority
EP
European Patent Office
Prior art keywords
resonator
rings
current
loops
paths
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP91920669A
Other languages
German (de)
English (en)
Other versions
EP0554388B1 (fr
EP0554388A4 (en
Inventor
Joseph Murphy-Boesch
Ravi Srinivasan
Lucas Carvajal
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Fox Chase Cancer Center
Original Assignee
Fox Chase Cancer Center
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Fox Chase Cancer Center filed Critical Fox Chase Cancer Center
Publication of EP0554388A1 publication Critical patent/EP0554388A1/fr
Publication of EP0554388A4 publication Critical patent/EP0554388A4/en
Application granted granted Critical
Publication of EP0554388B1 publication Critical patent/EP0554388B1/fr
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/32—Excitation or detection systems, e.g. using radio frequency signals
    • G01R33/34—Constructional details, e.g. resonators, specially adapted to MR
    • G01R33/34046—Volume type coils, e.g. bird-cage coils; Quadrature bird-cage coils; Circularly polarised coils

Definitions

  • the present invention relates to an improvement in nuclear magnetic resonance (NMR) apparatus and, more particularly, to resonators for transmitting and/or receiving radio frequency (hereafter designated RF) signals characteristic of signals emitted by nuclei in NMR analyses. More specifically, the resonator of the present invention provides a high signal to noise ratio and enhanced control of the RF (B j field distribution with improved homogeneity in a selected region of interest.
  • NMR nuclear magnetic resonance
  • RF radio frequency
  • the NMR technique is based upon the magnetic properties of nuclei containing odd numbers of protons and neutrons. These nuclei possess an angular momentum related to the charge thereof. The magnetic moment is directed along the spin axis of each nucleus. When placed in a strong and generally homogeneous static magnetic field, designated B 0 , the nuclei either align with or against the applied field and precess with a common sense about the applied field. The precessional angle of a nucleus may be changed by absorption of electromagnetic energy through a phenomenon known as nuclear magnetic resonance, NMR, which involves impressing upon the nuclei a second rotating magnetic field, designated B]_, of frequency to match that of their normal precession.
  • NMR nuclear magnetic resonance
  • a bias or gradient in the normally homogeneous B Q field is introduced across the sample for the purpose of spatially encoding information into the NMR signals. Images are later reconstructed from the information contained within this data, forming the basis of NMR imaging, a technique now widely used in medical diagnostics.
  • the homogeneity of the BQ field is reflected in the quality of its proton images, that is more homogeneous fields produce images with less distortion intensity.
  • the Bi field for transmitting to the sample is derived most efficiently from a resonant RF coil placed in proximity to the sample and connected to the RF transmitting apparatus. Either the same or a second RF coil may be connected to the RF receiving apparatus to receive the NMR signals, which are induced in the coil by the precessing magnetism of the nuclei. Free induction signals from chemically shifted nuclei and from samples with B Q field gradients impressed upon them are normally received with a single-resonant coil tuned to the Larmor frequency of the nucleus.
  • the Bj_ field generated by this receiving coil must be homogeneous over the sample to produce more uniform spectral measurements and images.
  • a linear oscillating field such as produced by a simple resonant coil, can be cast as the sum of two circularly polarized components of equal amplitude.
  • Circularly polarized magnetic field can be produced which matches the precessional motion of the nuclei.
  • Circularly polarized coils are similar to crossed-coil double-tuned probes in that two resonant circuits require tuning. They differ, however, in that being of the same frequency, they require a high degree of electrical isolation to operate independently, as will be shown later.
  • the present invention has a primary purpose to provide a volume resonator capable of operating in circularly polarized mode (spatial quadrature) at any given NMR" * frequency.
  • Fig. 1 is a developed partial schematic for a prior art Low Pass birdcage coil
  • Fig. 2a is a perspective view of a preferred physical embodiment of a sixteen segment four-ring low-pass RF resonator in accordance with the present invention
  • Fig. 2b is a similar view of the resonator of Fig. 2a as a schematic circuit, but showing the location of capacitors and input/output coupling points;
  • Fig. 3a is a schematic diagram showing for analysis the capacitors of one of the outer Low Pass volume resonator capable of operating independently;
  • Fig. 3b is a portion of the developed schematic equivalent circuit of the capacitor containing cylindrical resonator meshes of Fig. 3a;
  • Fig. 3c is an individual repetitive circuit unit of the circuit of Fig. 3b;
  • Fig. 4b is an individual repetitive circuit unit of the circuit of Fig. 4a;
  • Fig. 4c is a plot of reflected power vs. frequency response of the circuit of Figs. 4a and 4b using an inductive coupling loop placed over one outer structure
  • Fig. 4d is a plot of frequency of the same circuit using an inductive coupling loop placed in the center of the resonator
  • Fig. 6a shows the orientation of the counter-rotating fundamental modes of the resonator of the invention operating near 22 MHz
  • Fig. 6b shows the orientation of the co-rotating fundamental modes of the resonator of the invention operating independently near 25 MHz;
  • Fig. 7a illustrates the sinusoidally distributed counter-rotating currents of one linear mode in the outer structures and the electric potentials of similar polarity developed therein across the inner Structure due to the currents in the two outer structures.
  • Fig. 7b illustrates the sinusoidally distributed co-rotating currents of one linear mode in the outer structures and the electric potentials of opposite polarity developed therein across the inner Structure thereby allowing current to flow across the conductive segments of the inner structure.
  • Fig. 8a illustrates normalized RF B ⁇ fields along the Z axis for the prior art coil shown in dashed lines and for this resonator of the present invention shown in dark lines.
  • Fig. 8b illustrates normalized RF B fields along the X,Y axis for the prior art coil shown in dashed lines and for this resonator of the present invention shown in dark lines.
  • Fig. 9 depicts the preferred physical coil form of the present invention in a planar developed representation showing mounting supports utilized in the preferred embodiment
  • Fig. 10 is a schematic partial development of the preferred embodiment circuit from Figs. 2b and 9 to provide direct comparison with the following figures;
  • Fig. 11 depicts a partial planar schematic embodiment of the resonator of the present invention which may be referred to as the four ring single ended high pass volume resonator;
  • Fig. 12 depicts yet another partial planar schematic embodiment of the resonator of the present invention which may be referred to as the four ring band pass volume resonator;
  • Figs. 13, 14 and 15 are schematic partial developments similar to Fig. 10 showing alternative circuit configuration embodiments of the present invention.
  • the applied field B 0 by convention is considered to be directed in and defining the Z-direction for NMR.
  • RF coils that are a part of a simple resonant circuit, have a single distribution of current oscillating in phase and produce a linearly polarized RF magnetic field, Bi, at each point in the sample.
  • the linearly oscillating component of B ⁇ transverse to the applied field B 0 that is B ⁇ xv , nutates the nuclei in a predetermined manner during transmission.
  • the coil receives signals from nuclei of the sample with a profile weighted by the magnitude of B ⁇ xv . See D.I. Hoult and R.E. Richards, J . Magn.
  • transmitter power is divided equally between left-hand and right-hand circular polarizations. Since only one of these polarizations matches the precessional motion of the nuclei, a factor of two reduction in required transmitter power can be achieved by direct generation of the single polarization. By reciprocity, the signal detected from the sample will likewise double. Noise from the two linear channels used to detect each linear polarization, on the other hand, is uncorrelated and therefore increases by a factor of square root of two upon being summed, for example within a quadrature hybrid. A net increase of square root of two in sensitivity can thus be obtained by use of a circularly polarized coil.
  • the resonator may be driven in circularly polarized mode either capacitively across the input/output coupling points as shown in Fig. 2b or inductively with coupling loops like the one shown •20 in Fig. 3a, spaced 90 degrees apart.
  • the ports couple only to their respective modes, and a high degree of electrical isolation between the ports is achieved. Under the condition of weak coupling between matched ports, the
  • ratio of the output voltage to the input voltage at the other port is ⁇ Q/2 r where K is magnetic coupling coefficient and Q is- the circuit Q.
  • K is magnetic coupling coefficient
  • Q is- the circuit Q.
  • this value should be 0.03 (-30 dB) or greater for superior transmitter/receiver performance.
  • Fig. 1 is a partial planar schematic of a birdcage coil. It effectively involves a pair of
  • Fig. 1 35 closed ring conductors 10 and 12 having uniformly spaced identical connecting conductors 14, each of which contains a bypass capacitor 16.
  • the coil illustrated in Fig. 1 is typically utilized in NMR studies of the head. Owing to the sinusoidally distributed currents in the straight segments of the coil, the radial homogeneity of the birdcage coil is greatly improved over saddle coils, especially in annular regions of the coil interior close to the coil conductors. See U.S. Patent No. 4,694,255. Homogeneity is further improved in the latter regions by increasing the number of straight segments 14, for example from eight to sixteen.
  • the RF Bi field profile along the longitudinal coil axis is Gaussian like in shape, falling off from its maximum near the coil center to about half its maximum near the ends of the coil.
  • RF B ⁇ field homogeneity along the coil axis is improved by increasing the coil length. Greater field homogeneity is obtained at the expense of coil sensitivity, however, since the extra length of the coil will incur greater resistive losses, both in the sample, or tissue, and in the conductors of the coil. See Bottomley et al., Magn. Reson. Med. 9, 319-336 (1988). This loss in coil sensitivity is not acceptable for our in-vivo NMR studies of the head, where low signal to noise ratio data from multiple regions of the head provide useful information.
  • the preferred embodiment of the RF resonator of the present invention may be referred to as a "four ring" birdcage resonator.
  • the geometry closely resembles that of our RF resonator disclosed in our co-pending United States patent application Serial No. 561,898 filed August 2, 1990.
  • the four conductive rings 59, 56, 58, 60 which are coaxial, of the same diameter, and spaced along the coil axis to define the cylinder of the "birdcage", are needed to achieve the aims enumerated above.
  • a coil is supported on a thin walled tube 40 of a preferably transparent insulating material. It is also preferably capable of being made of sufficiently large size to permit a patient's head to be placed within the tube.
  • the outer cylindrical surface of the tube is covered with a foil or sheet 42 which has through it a plurality of rectangular or oblong openings 44, 46, 48.
  • the openings are aligned along elements of the cylinder and the central opening 44 is of the same width circumferentially and in general, but not necessarily, of the same length axially from openings 46 at one end and opening 48 at the other.
  • the openings 44 , 46 and 48, respectively, are arranged around the circumference equally spaced from one another.
  • the openings are not only equally spaced from one another and uniform in size, but the conductive segments 50, 52 and 54, respectively, between them are uniform.
  • Between openings 44 and 46 is circumferential continuous ring 56 and between openings 44 and 48 continuous ring 58. At the outside edges are continuous rings .59 and 60, respectively.
  • Strip segments 50 between continuous rings 56 and 58 are uninterrupted but strip segments 52 and 54 are interrupted by gaps which are similarly bridged by capacitors 62 and 64, respectively.
  • the conductors may be made up of individual conductive elements which are wires, conducting tubes, flat conducting tapes or any combination thereof.
  • a pair of low-pass outer structures resonant at a selected frequency are separated by non-resonant loops defined by the two inner rings 56, 58, and the conductive segment connections 50 between them.
  • the conductive segments are parallel to the longitudinal axis of the coil and to one another and are evenly spaced around the cylinder they define with the rings.
  • the section of the coil formed by the inner, evenly spaced pair of conductive rings and the evenly spaced conductive segments connecting them will hereafter be referred to as the "inner structure".
  • Two outer resonant structures share common rings 56,58 with the inner structure.
  • Extending between inner rings 56,58 and outer rings 59,60 are conductive segments 52,54.
  • the segments 52,54 are parallel to the longitudinal axes of the coil and to one another and are evenly spaced around the cylinder they define with the rings.
  • the capacitors 62,64 are added in the conductive segments 52,54 respectively. Using only one set of capacitors 62 or 64 at a time, each "outer structure" of the coil resonates with the same number and distribution of modes with each mode resonating approximately at the same frequency.
  • Conductive segments 50 of the inner structure and 52,54 of the outer structure are parallel to the longitudinal axis of the resonator and in this embodiment are in line with one another.
  • Input and output coupling to the resonator may be either inductive or capacitive.
  • capacitive coupling for circularly polarized operation is accomplished using input/output terminals 63a or 63b or both at a selected frequency and input/output terminals 65a or 65b or both at the same frequency 90° out of phase. Operation in linearly polarized mode is accomplished using only one set of terminals 63a, 63b or 65a, 65b.
  • FIG. 2b A better understanding of the coil depicted schematically in Fig. 2b can be acquired by study of the lumped-element equivalent circuits of each of the outer structures shown in Figs. 3a-3c and the composite structure formed by joining the structures together seen in Figs. 4a and 4b.
  • the lumped element inductances are shown as the number of the segment in which they appear with the suffix L, i.e., 50L, 56L, 58L, 59L, 60L or in those cases where a capacitance splits a segment by L and L' , i.e., 52, 52L' , 54 and 54L. Referring to Fig.
  • each outer structure is capable of operating independently as a low-pass volume resonator, where the capacitors 64 are mounted across a gap in each conductive segment 54. In the absence of capacitors 62, no current will flow in the opposite outer structure.
  • Fig. 3b is the lumped-element equivalent circuit of one outer resonant structure of the four ring low-pass volume resonator shown in Fig. 3a.
  • the complete circuit of the outer structure is formed by joining terminals A and B to terminals A' and B' , respectively.
  • the ladder network is made up of sixteen repeat circuit units shown in Fig. 3c.

Landscapes

  • Physics & Mathematics (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)
  • Control Of Motors That Do Not Use Commutators (AREA)
  • Coils Or Transformers For Communication (AREA)

Abstract

Résonateur RF destiné à être utilisé dans un dispositif de résonance magnétique nucléaire. Ledit résonateur est monté sur un support diélectrique suffisamment grand pour permettre l'insertion de la tête d'un patient. Le résonateur comporte quatre voies annulaires de courant de haute fréquence réparties axialement (56, 58, 59, 60) sur le support (40), et au moins quatre voies de courant de haute fréquence (50) interconnectent chaque paire de voies annulaires adjacentes. Dans un mode préféré de réalisation, au moins seize segments (50) parallèles également espacés connectent chaque paire d'anneaux et sont alignés les uns par rapport aux autres. Les segments d'anneaux situés entre les segments parallèles forment des boucles avec les segments parallèles. Au moins un condensateur discret (62, 64) est placé dans chaque boucle de courant extérieure et interrompt ainsi la boucle. Lesdits condensateurs sont toutefois placés de manière à n'interrompre aucune boucle intérieure. Il s'ensuit que chaque boucle n'a effectivement aucune capacité et devient résonante aux radiofréquences.
EP91920669A 1990-10-25 1991-10-10 Resonateur de volume rf pour resonance magnetique nucleaire Expired - Lifetime EP0554388B1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US07/603,947 US5212450A (en) 1990-10-25 1990-10-25 Radio frequency volume resonator for nuclear magnetic resonance
US603947 1990-10-25
PCT/US1991/007476 WO1992008145A1 (fr) 1990-10-25 1991-10-10 Resonateur de volume rf pour resonance magnetique nucleaire

Publications (3)

Publication Number Publication Date
EP0554388A1 true EP0554388A1 (fr) 1993-08-11
EP0554388A4 EP0554388A4 (en) 1993-12-08
EP0554388B1 EP0554388B1 (fr) 1997-01-08

Family

ID=24417554

Family Applications (1)

Application Number Title Priority Date Filing Date
EP91920669A Expired - Lifetime EP0554388B1 (fr) 1990-10-25 1991-10-10 Resonateur de volume rf pour resonance magnetique nucleaire

Country Status (8)

Country Link
US (1) US5212450A (fr)
EP (1) EP0554388B1 (fr)
JP (1) JPH06502491A (fr)
AT (1) ATE147516T1 (fr)
CA (1) CA2094714A1 (fr)
DE (1) DE69124103D1 (fr)
IL (1) IL99772A (fr)
WO (1) WO1992008145A1 (fr)

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Also Published As

Publication number Publication date
IL99772A (en) 1994-10-21
ATE147516T1 (de) 1997-01-15
WO1992008145A1 (fr) 1992-05-14
EP0554388B1 (fr) 1997-01-08
IL99772A0 (en) 1992-08-18
JPH06502491A (ja) 1994-03-17
EP0554388A4 (en) 1993-12-08
US5212450A (en) 1993-05-18
CA2094714A1 (fr) 1992-04-26
DE69124103D1 (de) 1997-02-20

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